Lap joints are created when two or more aircraft skin panels are joined, and a portion of one panel (e.g., an inner panel) is overlapped by portion of another panel (e.g., an outer skin panel). The term “lap joint,” as used herein, refers both to longitudinal joints, as formed when outer (e.g., an upper longitudinal panel) and inner (e.g., a lower longitudinal panel) fuselage skin panels are joined, and to circumferential or butt joints, as formed when two curved skin panel assemblies are joined by a structural panel (e.g., a splice plate). Similarly, the term “inner panel,” as used herein, may refer to any structural panel (e.g., a splice plate or an inner fuselage skin panel) that is at least partially overlapped at a lap joint. Lap joint panels are typically joined together utilizing an anti-corrosive sealant and rows of rivets disposed proximate the overlapping edge of the outer skin panel.
It has been discovered that the surface of inner panels may be scratched proximate the lap joints during routine maintenance. For example, very fine and shallow scribe lines may be created by certain tools during removal of excess lap joint sealant. The scribe lines may be blended out by abrasively removing a shallow volume of material along the surface of the panel, providing that scratches are visible and accessible and that the scratched skin panel is sufficiently thick. Alternatively, it may be necessary to remove a portion of the panel and install and install a replacement panel such as a repair doubler. In any event, the detection of scribe lines and other surface imperfections can be an important aspect of aircraft maintenance.
For practical maintenance cost reasons, it should be possible to detect scribe lines and other surface imperfections of aircrafts panel in a non-laboratory environment. Traditional detection techniques include visual inspection and the use of low and high frequency eddy current inspection equipment. In practice, however, the fuselage skin of an aircraft usually has surface irregularities that can mask the test signals generated by conventional eddy current inspection technologies. In addition, conventional eddy current inspection technologies may not be sensitive enough to detect extremely shallow imperfections. In this regard, typical high frequency eddy current detection systems can detect surface irregularities that are at least 0.020 inch deep. Such conventional systems are not suitable for the detection of the fine scribe line and cracks described above, which can be as shallow as 0.003 inch deep.
Accordingly, it is desirable to have a system for rapidly detecting very shallow imperfections in the fuselage skin of an aircraft in a manner that is immune to normal or acceptable surface irregularities. In addition, it is desirable to have a field-deployable eddy current detection system that is sensitive enough to detect very shallow irregularities that would otherwise remain undetected by conventional eddy current systems. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.